Configuring, Modifying, and Monitoring the Unit
35
Topology Change Notification BPDU
A non-Root interface sends a Topology Change Notification BPDU (TCN BPDU) over its Root
interface any time it believes that the network topology has changed.
The following circumstances can cause this to happen:
• A Root interface gives up its Root status and the topology state is changing.
• A switch receives a TCN BPDU from another unit via a non-Root interface.
• An interface's state changes (it enters the Forwarding state). When a Root interface
receives a TCN BPDU, it sets a Topology Changing flag in its Hello BPDU for an interval
that is twice the length of the Forward Timer. When a non-Root interface receives a Hello
BPDU with this flag set, it quickly ages its dynamic filter s so that it can relearn its filters
based on the new topology.
Using STP to Manage Redundant Links
Multiple switches can be connected to form a larger switched network, without using STP the
connections between the switches would form a loop. STP should be used to block one of the
connections. If the primary connection fails for some reason, STP reconfigures the topology and
direct all inter-switches traffic on the redundant link.
Spanning Tree and VLAN restrictions
If you want to use STP and VLAN you should know how the implementation of STP in our
products operates. One instance of STP runs in the management VLAN. So the consequence is
that STP does not resolve loop in VLAN to which it does not belong. If you want to use the
redundancy facility you should use Trunking/EtherChannel feature instead.
Spanning Tree Commands
To view the Spanning Tree commands, enter sp-tree at the CLI prompt. The following are the
available port commands.

Enabling/Disabling STP

set-stp enable
Enabling or disabling Spanning Tree does not take effect until the SNMP agent is reset, via
warm-reset, cold-reset or a power cycle. The default is disabled.
get-stp
The get-stp command retrieves the current state of Spanning Tree.
Example
SYS_console> get-stp
Running spanning tree engine is disable
Next session of the Spanning Tree engine will be disabled.
SYS_console>